[LAU] The future of realtime Linux

Gene Heskett gheskett at wdtv.com
Fri Nov 15 08:52:10 UTC 2013

On Friday 15 November 2013 03:26:18 Ralf Mardorf did opine:

> http://lwn.net/Articles/572740/
This concerns the linuxcnc folks, including me.

However PREEMPT-RT isn't the only method and in fact is far from the best 
to gain some high speed IO.

Paolo has finally understood that RTAI needs to be brought up to date, and 
I am hearing rumors that it has even been built on a 3.10 kernel.

Alternatively the Xenomai patches are working reasonably well, ISTR I did a 
test build of 3.8.3 +Xenomai on my lappy, and while it could have run a 
machine, the lappy doesn't have a parport, and its cpu doesn't have even 
the iron an Atom board has, so that got wiped when I put Mate 14 on it.

Right now, the "gold card" motherboard for us running machinery in real 
time is from the Atom cpu camp, where latencies for a 50 kilohertz 
base_thread are the usual bell curve shaped, with 99.9% of the tasks 
completed within 2 u-secs of scheduled, worst case perhaps 7 u-secs when 
the machine is doing something else.  Like opening a new screen in firefox.
This is running that 2.6.32-122-rtai kernel we've been using since 8.04 
ubuntu LTS.  But thats a poor kernel for everyday use, no PAE.

From the development work going on in linuxcnc right now, its looking like 
we may have found our next platform in the BeagleBone Black when a suitable 
bit/pin oriented cape is shipping.  This of course won't do for audio since 
we do IO a bit at a time independent of any other bit in its copious IO 
space.  Several of them are running production milling machines right now 
using homemade IO hardware.  For a $45 USD cost, board about the size of a 
credit card, thats amazing.

Cheers, Gene
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